Ed is absolutely right AGAIN.
I have owned Hasselblad since 1976. I originally owned 4 but now just have a CM and an ELM. I would definately recommend this system over any other. Virtually anything they make fits. No built in obsolescence. The cameras I bought in 1976 work the same they did the day I bought them. And I have used them a miniumum of 4 times a week.
I would stay away from the C lenses and don't but anything older than CF as they are running out of parts for the older lenses. If it's a silver lens don't buy it.
If possible buy a 503 with the 60 series lenses, and you will never be sorry.
The problem you could find with Rollei compared to Hasselblad is that there are hundreds of thousands of Hasselblads around and if you want to add on or upgrade you have a far better chance with Hasselblad.